The brief
Mr Julien's Golf was constantly low on coolant with intermittent overheating, the temperature climbing now and then. He brought it in, which is exactly right, coolant that disappears is going somewhere and an overheating engine can do real damage. On this engine the water pump and the thermostat are built into one housing assembly, a plastic unit bolted to the engine. The pump's seal weeps coolant and its bearing wears, the thermostat can stick, and the plastic housing itself can crack, so a tired assembly leaks coolant and stops circulating it properly, hence the low level and the intermittent overheat. You don't patch a cracked housing or rebuild the pump on the car, so the whole water pump and thermostat housing assembly comes as one and gets replaced.
The diagnosis
A pressure test on the cooling system pinpointed it, the water pump and thermostat housing assembly was weeping and the pump bearing was rough, with the impeller not moving water cleanly, which is the low coolant and the overheating. The radiator, the hoses, the expansion tank and the rest of the system held fine. That's an assembly replacement, with a fresh seal, rather than chasing a leak in tired plastic and a failing pump.
The work
The cooling system was drained, the old water pump and thermostat housing assembly removed, and a new genuine VW-spec assembly fitted with a fresh seal and the drive belt set back up properly. The system was refilled with the correct VW coolant, the air bled out the proper way so no pockets were left, and held under pressure to confirm the seals were dry. A road test confirmed the gauge sat steady through traffic and at speed with no overheating and no noise, and the level stayed put.
The outcome
No more coolant loss, the level holding between checks, the gauge steady, the engine warming up on time, and no overheating. The Golf went home with the cooling system circulating properly again. A tired water pump and thermostat housing only leaks worse and circulates less, and the failure is an overheat that can take the head gasket with it, so changing the assembly kept it to a tidy job.
